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15
Evaluating Your Measurement Results
Problem Solving
Table 15-4 Actions to Eliminate Spurs
Spur Sources
Description
Internal
Potential spur sources within the
measurement system include the phase noise
system, the unit-under-test, and the reference
source. Typical system spurs are –120 dBc,
and they occur at the power line and system
vibration frequencies in the range of from 25
Hz to 1 kHz, and above 10 MHz.
External
Spur sources external to the system may be
either mechanical or electrical. When using
the Phase Lock Loop measurement
technique, the system's susceptibility to
external spur sources increases with
increases in the Peak Tuning Range set by the
VCO source.
Electrical
Electrically generated spurs can be caused by
electrical oscillation, either internal or
external to the measurement system. The list
of potential spur sources is long and varied.
Many times the spur will not be at the
fundamental frequency of the source, but may
be a harmonic of the source signal. Some
typical causes of electrical spurs are power
lines, radio broadcasting stations, computers
and computer peripherals (any device that
generates high frequency square waves), and
sum and difference products of oscillators
that are not isolated from one another in an
instrument such as a signal generator.
Mechanical
Mechanically generated spurs are usually at
frequencies below 1 kHz. The source of a
mechanically generated spur is typically
external to the measurement system.
Small Angle Line
15-16
E5500 Phase Noise Measurement System Version A.02.00
Caution must be exercised where
density of the phase modulation
criterion. Refer to
Figure
correct; above the line,
used to accurately represent the phase noise of the signal. To
Recommended Action
If you do not have a plot of the system's noise
and spur characteristics, perform the system
Noise Floor Test. If you suspect that the
unit-under-test or the reference source may
be the spur source, check each source using a
spectrum analyzer or measuring receiver
(such as an Agilent 8902A). Also, if additional
sources are available, try exchanging each of
the sources and repeating the measurement.
Shorten coax cables as much as possible
(particularly the Tune Voltage Output cable).
Make sure all cable connections are tight. It
may be possible to identify an external spur
source using a spectrum analyzer with a
pick-up coil or an antenna connected to it.
The frequency of the spur and patterns of
multiple spurs are the most useful parameters
for determining the source of spurs. The spur
frequency can be estimated from the graph, or
pinpointed using either the Marker graphic
function which provides a resolution of from
0.1% to 0.2% or by using the spur listing
function.
Try turning off or moving fans, motors, or
other mechanical devices that oscillate at a
specific frequency. (Temporarily blocking the
airflow through a fan may alter its speed
enough to discern a frequency shift in a spur
that is being caused by the fan.)
L(f)
is calculated from the spectral
S
(f)
/2 because of the small angle
φ
15-12. Below the line, the plot of
L(f)
is increasingly invalid and Sf(f) must be
Document Part No. E5500-90024 Ed. 1.0
L(f)
is
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